1. Field of the Invention
The present invention relates to a coordinate input apparatus, and more particularly, to a coordinate input apparatus which detects by means of a vibration sensor disposed on a vibration transmission plate, a vibration inputted from a vibration pen so as to determine the coordinates of the vibration pen on the vibration transmission plate.
2. Description of the Related Art
A coordinate input apparatus shown in FIG. 13, which is, for example, disclosed in U.S. Pat. No. 4,931,965, is known in the prior art. The apparatus of FIG. 13 accepts coordinates input from a vibration pen 73 in an input tablet of a transmission plate 78. The input coordinate information is output to an information processing apparatus, such as a personal computer or the like, to which the coordinate input apparatus is connected.
The vibration pen 73 is a pen for generating vibration waves in the transmission plate 78 which transmits the vibration waves. The vibration pen 73 comprises a vibrator 74, a horn 75 and a supporting body therefor. Reference numeral 72 denotes a pen driving circuit; reference numeral 76 denotes a vibration sensor for detecting vibration waves transmitted through the transmission plate 78; and reference numeral 77 denotes a reflection preventing material for preventing reflection in an end surface of the transmission plate 78.
In a case where a transmission time Tp based on a phase speed Vp is measured and a distance r is computed, the distance r from an instructed point to the vibration sensor 76 becomes the following: EQU r=n.multidot..lambda.+vp.multidot.Tp (30)
where .lambda. is a wavelength of the vibration waves and n is an integer. The n of equation 30 can be determined from the following equation: EQU n=int [(VgTg-VpTp)/.lambda.+0.5] (31)
where Vg is a group velocity and Tg is a transmission time corresponding to the group velocity.
Since the group velocity Vg and the phase speed Vp can be considered to be constants unique to the material used for a propagation material, Tg and Tp are measured to determine the distance. Vibration wave detection circuits 1, 2, 3 denoted as elements 83 to 85, latch circuits 1, 2, 3 denoted as elements 86 to 88, and a timing counter 79 constitute a circuit for measuring Tg and Tp.
In the above-described construction, a control apparatus 71 drives the vibration pen 73 via drive circuit 72 and makes the timing counter 79 begin counting from zero.
Vibration waves generated by the vibration pen 73 reach the vibration sensor 76 after a lapse of transmission times Tg and Tp based on the group velocity Vg and the phase speed Vp. The vibration waves are converted into electrical signals by the vibration sensor 76 and reach vibration wave detection circuits 1, 2 and 3 (elements 83 to 85) after passing through prestage amplifiers 1, 2 and 3 (elements 80 to 83). The vibration wave detection circuits detect a point on a vibration waveform based on the group velocity and the phase velocity and outputs Tg and Tp detection signals to latch circuits 1, 2 and 3 (elements 86 to 88).
The latch circuits 1, 2 and 3 (elements 86 to 88) use these Tg and Tp detection signals as triggers to read in the count value of the timing counter 79.
The control apparatus 71 computes each distance from the vibration sensor 76 to an input point of the pen on the basis of equations (30) and (31) from transmission times Tg and Tp measured in the above way and performs a geometric calculation thereof to obtain coordinate values.
At this time, as is apparent from equation (30), constants used for detecting the distance r are a wavelength .lambda. (=VpT) and the phase speed Vp. These constants are determined on the basis of the frequency f (=1/T) of the detected vibration waves and the phase speed Vp.
However, needless to say, the wavelength .lambda. and the phase speed Vp in equation (30) must be determined accurately in order to obtain highly accurate coordinates in the above-described prior art.
It is generally known that the velocity Vp of plate waves which propagate through a propagation body (the vibration transmission plate 78) depends upon the frequency of the plate waves. The vibration input pen 73 for generating plate waves obtains mechanical energy by applying a high-frequency voltage to a vibrator 74, which is composed of, for example, piezoelectric elements in an input pen, and causes a propagation body to generate plate waves through a member, such as a horn.
In contrast to a driving frequency of an applied voltage, the response frequencies of waves input to the propagation body differ from vibration input pen to vibration input pen due to variations in the mechanical characteristics (e.g., resonance characteristics) of piezoelectric elements or to variations in the mechanical characteristics of a member, such as a horn.
Therefore, it is necessary to know the characteristics of individual input pens to realize a coordinate input apparatus with a high degree of accuracy.
Also, since constants Vp and .lambda. depend upon the frequency of plate waves, they also depend upon the frequency of the pulses by means of which the piezoelectric elements are driven. Therefore, in mass production, to eliminate variations in the driving frequency, the variations of individual circuits must be eliminated and circuits having no variations of those of electronic parts must be used. From this viewpoint, the conventional construction has a problem in that an increase in cost is required to achieve higher accuracy.
Even if low-cost circuits having a high degree of accuracy are used, the driving frequency must be measured for each circuit. Inspection increases the cost in the same manner as input pen variations, and thus the circuits are not suited for mass production.
In actual use, a horn member, which is a pen tip of the input pen, is worn down over time due to the contact and sliding with the input surface (i.e., the transmission plate) .
Therefore, the mechanical characteristics of the horn member change with the amount of wear. As a result, a frequency outputted by the pen tip changes with the amount of wear. There is a problem in that the set constants f, Vp and Vg change and accuracy decreases.
In the above-described prior art, the response in the pen tip with respect to a frequency at which piezoelectric elements are driven is not a single frequency spectrum, but contains various frequency components. Therefore, signal waveforms which are transmitted on the transmission plate and output from the sensor contain the various frequency components. Therefore, the cycle of the phase of the detected signal waveforms is not fixed. A detection point at a phase delay time tp based on the phase speed Vp changes according to the difference in levels of the detected signal waveforms caused by the changes in the input state of the pen.
That is, in FIG. 10, it is assumed that a detection point of Tp in a certain state is at a point corresponding to a delay time Tp2. At this time, a calculated distance 10 becomes the following from equation (30): EQU 10=n0.lambda.+VpTp2
n0 becomes the following from equation (31): EQU n0=int [(VgTg0-VpTp2)/.lambda.+0.5]
Assuming that, with the distance of the input pen being unchanged, for example, the writing pressure of the input pen changes and the level of the detected waveform changes, causing a Tp detection point to change to a point corresponding to a delay time Tp3, and a distance 10' calculated at this time becomes the following from equation (31): ##EQU1##
From equation (30), ##EQU2##
Here, due to the above-described problems, T1, T2, T3, T4 . . . Tn are not fixed. Accordingly, there is a problem in that, even if the same point is inputted, calculated results will differ due to a variation (for instance, the relation VpT2=.lambda. does not hold) in Tn when the Tp detection point deviates.